Nuclear reactors use the thermal energy produced through fission to produce energy. Typically, a coolant such as water flows about nuclear fuel elements contained within a reactor vessel under such a high pressure that it remains in liquid form at a temperature far above the normal boiling point. The coolant goes to a heat exchanger including a feedwater header, where it gives up heat to a secondary stream of water that turns to steam while the primary stream of coolant returns to the reactor vessel. The steam is then used to run a steam generator turbine. Alternatively, the pressure is adjusted so that steam generation occurs as the water passes over the fuel elements. In the latter case, the steam passes directly from the reactor vessel to one or more steam generated turbines and is then condensed by a condenser before returning to the reactor vessel.
Nuclear reactors are designed to operate safely without releasing radioactivity to the outside environment. Nevertheless, it is recognized that accidents can occur. As a result, the use of multiple barriers has been adopted to deal with such accidents. These barriers include the fuel cladding, the reactor coolant or steam supply system, and thick shielding. As a final barrier, the reactor is housed in a large steel containment building.
Containment buildings vary considerably in design from plant to plant. Many are vertical cylindrical structures covered with a hemispherical or shallow domed roof and with a flat foundation slab. Other containment buildings may be spherical in shape.
Containment buildings are often not visible since they are usually surrounded by a steel or concrete outer building that also include many non-essential plant support systems, structures and auxiliary buildings which need not be included within the containment building. Nevertheless, these other systems and auxiliary structures and buildings must be located in close proximity to the reactor containment building.
A containment building houses the entire primary system of a nuclear reactor including the reactor vessel, reactor coolant or recirculation systems, pumps, and steam generators. The containment building includes a number of compartments for the housing of auxiliary equipment, safety systems, and various other systems.
The containment building is designed and tested to prevent any radioactivity that escapes from the reactor from being released to the outside environment. As a consequence, the building must be airtight. In practice, it must be able to maintain its integrity under circumstances of a drastic nature, such as accidents in which most of the contents of the reactor are released to the building. It has to withstand pressure buildups and damage from debris propelled by an explosion within a reactor. It must past tests to show it will not leak even when its internal pressure is well above that of the surrounding air. Typically, a containment building is designed to sustain internal pressures in the range of 45 to 60 psig. However, much higher pressures, even exceeding 100 psig, may be sustained.
The containment building is also designed and tested to protect a reactor against outside forces. Such outside forces include natural or man-made forces such as earthquakes, floods, tornadoes, explosions, fires and even airplane crashes.
One of the major factors influencing containment building design and placement is economic, since a containment building is one of the most expensive structures of a nuclear power plant. Containment buildings are currently usually designed in accordance with site-specific requirements established for each nuclear power plant. Site-specific designs prevent the standardization of the containment building and further increase cost.
As a result of such expenses, it is desirable to maximize the amount of energy generated by such a plant. There is a current limit of approximately 1800 MW of thermal power heat generation from a single reactor. However, core stability is less than ideal at such a reactor unless substantial expensive modifications are made. As a result, in modern practice, a single reactor has been placed in a single containment building having an energy output on the order of approximately 850 to 1450 MW. The greatest core stability occurs at approximately 1100 MW of thermal power generation.